1. Field of the Invention
This invention generally relates to a phase-locked loop (PLL) frequency synthesis system and, more particularly, to a system and method for maximizing frequency lock stability in receiver utilizing a plurality of voltage controlled oscillators with overlapping frequency bands.
2. Description of the Related Art
Voltage controlled oscillators are commonly used in monolithic clock data recovery (CDR) units, as they're easy to fabricate and provide reliable results. Clock recovery PLLs generally don't use phase-frequency detectors (PFDs) in the data path since the incoming data signal isn't deterministic. PFDs are more typically used in frequency synthesizers with periodic (deterministic) signals. Clock recovery PLLs use exclusive-OR (XOR) based phase detectors to maintain quadrature phase alignment between the incoming data pattern and the re-timed pattern. XOR based phase detectors have limited frequency discrimination capability, generally restricting frequency offsets to less than the closed loop PLL bandwidth. This characteristic, coupled with the wide tuning range of the voltage controlled oscillator (VCO), requires CDR circuits to depend upon an auxiliary frequency acquisition system.
There are two basic PLL frequency acquisition techniques. The first is a VCO sweep method. During an out-of-lock condition, auxiliary circuits cause the VCO frequency to slowly sweep across its tuning range in search of an input signal. The sweeping action is halted when a zero-beat note is detected, causing the PLL to lock to the input signal. The VCO sweep method is generally used in microwave frequency synthesis applications. The second type of acquisition aid, commonly found in clock recovery circuits, uses a PFD in combination with an XOR phase detector. When the PLL is locked to a data stream, the PLL switches over to a PFD that is driven by a stable reference clock source. The reference clock frequency is proportional to the data stream rate. For example, if the data stream rate is D and the reference clock rate is R, then D α R. However, since the reference clock has only a few rate settings, it is unlikely that R is equal to the receive data rate. To create a reference equal to the data rate a fractional ratio of R must be used; such as D=n/d*R.
In this manner, the VCO frequency is held very close to the data rate. Keeping the VCO frequency in the proper range of operation facilitates acquisition of the serial data and maintains a stable downstream clock when serial data isn't present at the CDR input. When serial data is applied to the CDR, the XOR based phase detector replaces the PFD, and data re-timing resumes.
It is common for a PLL to use a divider in the feedback path, so that the PFD can operate at lower frequencies. In the simplest case, the divisor is a fixed integer value. Then, a frequency divider is used to produce an output clock that is an integer multiple of the reference clock. If the divider cannot supply the required divisor, or if the output clock is not an integer multiple of the reference clock, the required divisor may be generated by toggling between two integer values, so that an average divisor results. By placing a fractional divider (1/N) into this feedback path, a fractional multiple of the input reference frequency can be produced at the output of this fractional-N PLL.
However, it is difficult to determine a divisor, either fixed or averaged, if the frequency of the data stream is not known beforehand. For this reason, CDR devices are typically designed to operate at one or more predetermined data stream baud rates.
Conventional fractional-N frequency synthesizers use fractional number decimal values in their PLL architectures. Even synthesizers that are conventionally referred to as “rational” frequency synthesizers operate by converting a rational number, with an integer numerator and integer denominator, into resolvable or approximated fractional numbers. These frequency synthesizers do not perform well because of the inherent fractional spurs that are generated in response to the lack of resolution of the number of bits representing the divisor in the feedback path of the frequency synthesizer.
FIG. 1 is a schematic block diagram depicting an accumulator circuit capable of performing a division operation (prior art). As noted in “A Pipelined Noise Shaping Coder for Fractional-N Frequency Synthesis”, by Kozak et al., IEEE Trans. on Instrumentation and Measurement, Vol. 50, No. 5, October 2001, the depicted 4th order device can be used to determine a division ratio using an integer sequence.
The carry outs from the 4 accumulators are cascaded to accumulate the fractional number. The carry outs are combined to reduce quantization noise by adding their contributions are follows:contribution 1=c1[n]; contribution 2=c2[n]−c2[n−1];contribution 3=c3[n]−2c3[n−1]+c3[n−2];contribution 4=c4[n]−3c4[n−1]+3c4[n−2]−c4[n−3];
where n is equal to a current time, and (n−1) is the previous time, Cx[n] is equal to a current value, and Cx[n−1] is equal to a previous value.
FIG. 2 shows the contributions made by the accumulator depicted in FIG. 1 with respect to order (prior art). A fractional number or fraction is a number that expresses a ratio of a numerator divided by a denominator. Some fractional numbers are rational—meaning that the numerator and denominator are both integers. With an irrational number, either the numerator or denominator is not an integer (e.g., π). Some rational numbers cannot be resolved (e.g., 10/3), while other rational numbers may only be resolved using a large number of decimal (or bit) places. In these cases, or if the fractional number is irrational, a long-term mean of the integer sequence must be used as an approximation.
The above-mentioned resolution problems are addressed with the use of a flexible accumulator, as described in parent application Ser. No. 11/954,325. The flexible accumulator is capable of performing rational division, or fractional division if the fraction cannot be sufficiently resolved, or if the fraction is irrational. The determination of whether a fraction is a rational number may be trivial in a system that transmits at a single frequency, especially if the user is permitted to select a convenient reference clock frequency. However, modern communication systems are expected to work at a number of different synthesized frequencies using a single reference clock. Further, the systems must be easily reprogrammable for different synthesized frequencies, without changing the single reference clock frequency.
As noted above, modern communication systems are expected to operate at a number of frequencies. In some circumstances the communication signal frequencies are unknown (not predetermined). While it is relatively straight-forward to reacquire the phase of a signal if the signal frequency is predetermined, it is necessarily more difficult to reacquire phase if the frequency is unknown. These difficulties are compounded if the system is expected to work over a wide frequency range. In that case, a large number of VCOs are required to cover the entire range. However, VCO operating characteristics are subject to temperature variations and fabrication tolerances. If a VCO is selected that is operating at the edge of its band, it may be unable to properly track an incoming communication signal. Alternately, the system may develop hysteresis as it toggles between different VCOs. A VCO that is operating at the center of its band is likely to continue to track an incoming signal, even as the system temperatures change. Thus, the VCO required to synthesize frequencies at start-up, may not be the optimal VCO when the system is operating at a temperature. Likewise, different fabrication lots may require different optimal VCOs for locking to the same frequency.
It would be advantageous if a VCO could be selected that operates near the center of its frequency band, as part of the process for frequency locking to an input communication signal.